Hemoglobin & the Bohr Effect

What hemoglobin actually is

Hemoglobin is the protein that turns blood red and carries almost all the oxygen in your body. A single red blood cell packs roughly 270 million copies of it, and you have about 150 grams of hemoglobin per liter of blood. Each molecule is a tetramer — two alpha-globin chains and two beta-globin chains (written α2β2), totaling about 64,500 daltons and 574 amino acids. Tucked into a hydrophobic pocket of each chain sits a heme: a flat porphyrin ring with a single iron atom at its center. That iron, held in the ferrous (Fe²⁺) state, is the actual oxygen grip. Four chains, four hemes, four irons — so one hemoglobin molecule can carry up to four O2 molecules.

Dissolved oxygen alone would never be enough: water carries only about 0.003 mL of O2 per 100 mL of plasma per mmHg, so arterial plasma holds only ~0.3 mL O2/100 mL. Hemoglobin multiplies that roughly 70-fold. Fully saturated, each gram of hemoglobin binds about 1.34 mL of O2 (Hüfner's constant), giving normal arterial blood an oxygen content near 20 mL per 100 mL. Without hemoglobin, an adult heart would have to pump tens of liters per minute just to keep tissues alive.

How the S-shaped curve works

Plot oxygen saturation (the percentage of heme sites carrying O2) against the partial pressure of oxygen (pO2) and you get a sigmoid — an S. This is the single most important graph in respiratory physiology, and its shape is no accident. It is the signature of positive cooperativity: the four binding sites are not independent. When the first O2 binds, it makes the next ones bind more easily.

The molecular cause is an allosteric, two-state switch. Deoxygenated hemoglobin sits in the tense (T) state — a quaternary arrangement locked by a network of salt bridges between the subunits, with low oxygen affinity. When O2 binds an iron, it pulls that iron from slightly out-of-plane into the plane of the porphyrin ring (a movement of about 0.4 ångström). The iron is tethered to a "proximal" histidine (His F8), so moving the iron drags the histidine and its helix. That tug propagates to the subunit interfaces, breaks the T-state salt bridges, and flips the tetramer toward the high-affinity relaxed (R) state. Now the remaining sites bind O2 far more readily — affinity rises roughly 100- to 500-fold between the first and last binding events. The Hill coefficient, which measures cooperativity, is about 2.8 for human hemoglobin (a value of 1 would mean no cooperativity and a hyperbolic curve; 4 would be the theoretical maximum for four sites).

The payoff is in the steep middle of the S. In the lungs, where pO2 is about 100 mmHg, hemoglobin sits near the flat top and is ~98% saturated. In resting tissue, pO2 is around 40 mmHg, still ~75% saturated — so resting blood unloads only about a quarter of its oxygen. But in hard-working muscle, pO2 can fall to 20 mmHg or lower, landing on the steepest part of the curve where a tiny pressure drop releases a large fraction of bound O2. A hyperbolic carrier (like single-site myoglobin) could not do this — it would either hold on too tightly or release too soon.

The Bohr effect, step by step

The S-curve is not fixed in place. Four signals slide it left or right by changing how strongly hemoglobin favors the T versus R state. Three of them — CO2, H+ (acid), and temperature — push the curve right (lower affinity, easier unloading). This rightward shift is the Bohr effect, first reported by the Danish physiologist Christian Bohr in 1904 (father of physicist Niels Bohr).

1. Falling pH / rising H+. Active tissue produces lactic acid and CO2-derived carbonic acid. The extra protons bind to specific residues — notably the C-terminal His146 of the beta chains and the N-terminal valine of the alpha chains — and re-form the very salt bridges that stabilize the low-affinity T state. So acidity strengthens T and squeezes O2 off the hemes.
2. Rising CO2. CO2 lowers pH indirectly (via carbonic anhydrase making H+ and HCO3-), but it also acts directly: it reacts with the uncharged N-terminal amino groups of the globin chains to form carbamino groups (carbamates), adding negative charge that creates new salt bridges favoring T.
3. Rising temperature. O2 binding to heme is exothermic, so heat (as in exercising muscle, ~38–40 °C locally) shifts the equilibrium toward release, right-shifting the curve.
4. Rising 2,3-BPG. This one is technically separate from the classic Bohr effect but works the same direction. 2,3-bisphosphoglycerate, a glycolytic byproduct in red cells, lodges in the central cavity of the T-state tetramer and clamps it shut, lowering affinity.

Every one of these conditions is a fingerprint of metabolically busy tissue: it is hot, acidic, and CO2-rich. So the Bohr effect is exquisitely targeted oxygen delivery — hemoglobin automatically lets go of more oxygen precisely where demand is greatest, then re-loads when it returns to the cool, low-CO2, alkaline environment of the lungs. The mirror-image relationship — deoxygenated hemoglobin binding CO2 and H+ better — is the Haldane effect, which helps the same blood pick up the CO2 that tissue just made.

The players and conditions

• Heme & ferrous iron (Fe²⁺). The actual O2-binding atom. Oxidize it to ferric (Fe³⁺) and you get methemoglobin, which cannot carry oxygen; the enzyme cytochrome-b5 reductase normally keeps methemoglobin below ~1%.
• Proximal histidine (His F8) & distal histidine (His E7). The proximal His anchors the iron and transmits the conformational signal; the distal His sits over the binding pocket, hydrogen-bonds bound O2, and sterically discourages CO and other ligands.
• 2,3-BPG. Roughly equimolar with hemoglobin in red cells (~5 mM). Its concentration is the body's slow dial for oxygen affinity — it climbs over hours to days at altitude or in anemia.
• Carbonic anhydrase. One of the fastest enzymes known (turnover ~10⁶ per second), it interconverts CO2 and bicarbonate inside red cells, driving both CO2 transport and the H+ supply for the Bohr effect.
• Globin gene switching. Embryonic (ζ, ε), fetal (α2γ2, HbF), and adult (α2β2, HbA; α2δ2, HbA2) hemoglobins each have different affinities, tuned to their oxygen environment.

Hemoglobin vs myoglobin vs fetal hemoglobin

Property	Adult hemoglobin (HbA)	Myoglobin	Fetal hemoglobin (HbF)
Subunits	Tetramer (α2β2)	Monomer (single chain)	Tetramer (α2γ2)
Binding sites	4 hemes	1 heme	4 hemes
Curve shape	Sigmoidal (cooperative)	Hyperbolic (no cooperativity)	Sigmoidal, shifted left
P50	~26–27 mmHg	~2.8 mmHg	~19 mmHg
Hill coefficient	~2.8	1.0	~2.4–2.8
Bohr effect	Strong	Negligible	Weaker
2,3-BPG binding	Strong (lowers affinity)	None	Weak (γ replaces β; lacks His143)
Role	Bulk O2 transport in blood	O2 storage / buffer in muscle	Pull O2 across the placenta

The contrast with myoglobin is the cleanest way to see why cooperativity matters. Myoglobin's single site gives a hyperbolic curve that is already ~95% saturated at 20 mmHg — great for grabbing and holding oxygen in muscle, useless for a delivery vehicle that must let go. Fetal hemoglobin's leftward shift (lower P50) lets it strip oxygen from the mother's blood across the placenta, where its higher affinity beats the maternal HbA at the same pO2.

The numbers that matter

Quantity	Typical value	Note
Molecular weight	~64,500 Da	α2β2, 574 residues
O2 per molecule	4	One per heme iron
O2 bound per gram Hb	1.34 mL	Hüfner's constant
Blood Hb concentration	~150 g/L (M), ~135 g/L (F)	Anemia < 130 / < 120
Arterial pO2 / saturation	~100 mmHg / ~98%	Top of the curve
Mixed venous pO2 / saturation	~40 mmHg / ~75% (rest)	~25% extraction at rest
P50 (standard)	~26–27 mmHg	pH 7.4, 37 °C, normal 2,3-BPG
Arterial O2 content	~20 mL O2 / 100 mL	(1.34 × Hb × SaO2) + dissolved
O2 affinity (CO vs O2)	~210–250× higher for CO	Why carbon monoxide is lethal
Hb lifespan	~120 days	= red cell lifespan

Where it shows up: disease, altitude, and the clinic

• Carbon monoxide poisoning. CO binds the same heme iron 200-plus times more avidly than O2, forming carboxyhemoglobin. Worse, it bends the curve of the remaining sites leftward, so even partially poisoned blood can't release the oxygen it still carries. This is why CO is dangerous at concentrations far below those that would matter for an inert gas, and why treatment is 100% (or hyperbaric) oxygen to compete CO off.
• High-altitude acclimatization. On day one at altitude, low arterial pO2 means lower saturation. Within hours to days, red cells ramp up 2,3-BPG, right-shifting the curve to improve tissue unloading; over weeks, the kidney's erythropoietin (EPO) raises red cell mass. Native high-altitude populations (Tibetans, Andeans) show distinct genetic adaptations (e.g., EPAS1/HIF pathway variants).
• Anemia and stored blood. Chronic anemia raises 2,3-BPG to squeeze more O2 out of fewer carriers. Conversely, banked blood depletes 2,3-BPG within ~1–2 weeks, so freshly transfused cells temporarily hold oxygen too tightly until they regenerate it.
• Sickle cell disease. A single Glu6Val mutation in beta-globin makes deoxygenated (T-state) HbS polymerize into fibers that sickle the cell. Anything that promotes the T state — hypoxia, acidosis, fever, dehydration — triggers painful vaso-occlusive crises, which is why the otherwise-helpful Bohr right-shift becomes a hazard.
• Pulse oximetry. The finger clip reads oxygen saturation by comparing how oxy- and deoxyhemoglobin absorb 660 nm (red) and 940 nm (infrared) light. It works because the two states have visibly different colors — the same physics that makes arterial blood bright red and venous blood dark.
• Fetal-maternal oxygen transfer. HbF's leftward shift, driven mostly by its poor 2,3-BPG binding, lets the fetus extract oxygen from maternal blood across the placenta even though both share the same gas pressures.

Common misconceptions and pitfalls

• "A right shift is bad." No — a right shift means hemoglobin releases oxygen more readily, which is exactly what tissues want. It only looks "bad" on a saturation number; at the tissue, lower affinity is helpful. The same shift that lowers measured saturation improves delivery.
• "The iron changes oxidation state when it binds oxygen." It stays Fe²⁺ the whole time. Oxygen binds as molecular O2, not as oxide. If the iron actually oxidizes to Fe³⁺, you get methemoglobin, which cannot carry oxygen at all — a malfunction, not the normal cycle.
• "More CO2 in blood means less oxygen because they compete for the same site." They don't compete for the heme. CO2 mostly travels as bicarbonate and as carbamino groups on the globin amino-termini, not on the iron. Its effect on O2 is allosteric (the Bohr effect), not direct site competition. (Carbon monoxide, by contrast, does compete at the iron.)
• "Saturation and content are the same thing." Saturation is the percentage of occupied sites; content is the actual amount of O2 per volume of blood, which also depends on how much hemoglobin you have. A severely anemic person can be 100% saturated yet dangerously low on oxygen content.
• "The plateau means extra oxygen is wasted." The flat top is a safety feature: arterial saturation barely drops even if alveolar pO2 falls moderately (mild lung disease, modest altitude), so loading is robust. The information-rich action happens on the steep middle, at the tissues.
• "Cooperativity means the sites are physically connected channels." Cooperativity is conformational, not a tunnel between sites. Binding at one heme changes the whole tetramer's quaternary shape (T to R), which alters the others' affinity from a distance — classic allostery.